As semiconductor technology progresses, shrinking device dimensions has become an increasingly complex task. Complementing metrology tools, allowing similar improvements in measurement capabilities, are critical for the continual process of this development. Commonly, optical metrology is applied to test structures comprised of a repeating array of identical elements. Optical metrology can acquire highly accurate and precise information on the geometry and material properties characterizing these structures.
In many cases, the test structure configuration (array) is an exact replica of an integrated device, and the device characteristics can be directly inferred from the metrology information obtained on the test structure. When the device itself is comprised of repeating structures, e.g. as common for memory applications, metrology can be applied directly to the device region itself. However, it is often the case that the device is not comprised of an array structure (e.g. in logic applications). In these cases, optical metrology is applied to a periodic structure which is supposed to represent the actual device.
U.S. Pat. No. 6,813,034 describes a technique of analyzing isolated and aperiodic structures with simultaneous multiple angle of incidence measurements. This patent discloses a method for evaluating isolated and aperiodic structure on a semiconductor sample, using focusing of a probe beam from a coherent laser source onto the structure in a manner to create a spread of angles incidence. The reflected light is monitored with an array detector. The intensity or polarization state of the reflected beam as a function of radial position within the beam is measured. Each measurement includes both specularly reflected light as well as light that has been scattered from the aperiodic structure into that detection position. The resulting output is evaluated using an aperiodic analysis to determine the geometry of the structure.